1. Field of the Invention
The present invention relates to a transmitter and a method of transmitting and in particular, but not exclusively, for use in a telecommunications network.
2. Description of the Related Art
In known wireless telecommunication networks 2 such as illustrated in FIG. 1, the area covered by the network 2 is divided into a plurality of cells 4. Each cell 4 has associated therewith a base transceiver station 6. Each base station 6 is arranged to communicate with terminals located in the cell 8 associated with that base station 6. The terminals 8 may be mobile terminals which are arranged to move between the cells 4.
Each base station 6 in the GSM (Global System for Mobile communications) is arranged to transmit N frequencies out of M available frequencies C1 . . . CM, as illustrated in FIG. 2a. Each of the M frequencies is different and within the bandwidth allocated for signals transmitted by base stations. Each channel is divided into a plurality of sequential frames F one of which is shown in FIG. 2b. Each frame F is divided into eight slots S0 . . . S7. The GSM standard is a time/frequency division multiple access system (F/TDMA) and accordingly signals will be transmitted by the base station to different mobile stations in different time slots. In other words, the base station will transmit signals to different mobile stations in different time slots at the same frequency. N is usually much less than M.
With known base transceiver stations a separate transmitter circuit is generally provided for each different frequency. For illustrative purposes a known base transceiver station is shown in FIG. 3. For the purpose of clarity only the transmit part 10 of the base station 12 is shown in FIG. 3.
The transmit part 10 comprises N different transmit paths 14–14′, one path being provided for each frequency. For clarity only the arrangement of one path 14 is shown but it should be appreciated that each path 14–14′ has the same structure. Each path 14–14′ comprises a modulator 16 which modulates the signal to be transmitted. The modulated signal is output to an amplifier 18 which amplifies the modulated signal. The output of the amplifier 18 is then filtered by a bandpass filter 20 which removes or attenuates unwanted noise and other emissions such as spurious signals and intermodulation products.
The output of each transmit path 14–14′ is connected to the input of a combiner 22 which combines the signals from each path 14–14′ to provide a multicarrier signal. The output of the combiner 22 is connected to an antenna 24 which transmits the multicarrier signal containing each of the N different channels to the terminals in the cell served by the base station.
It is desirable to reduce the number of transmit paths to one as this would significantly reduce the costs involved. However, if a single amplifier were to be used with a number of channels at the same time, that amplifier would need to be very linear to prevent the leakage of power into adjacent channels. The leakage of power into adjacent channels is undesirable in that a reduction in system capacity and/or a reduction in signal quality results. Linear amplifiers are known but even these amplifiers are not linear when operated near the peak capacity of the amplifier.
Several methods have been proposed for ensuring that amplifiers are linear. It is known, for example, from International Patent Application No. WO 97/30521 to use predistortion in a transmitter to try to ensure linearity. Predistortion is used to try to ensure that a transmitter is linear. Amplifiers introduce non linearity. With predistortion, a signal is distorted prior to being input to the amplifier which causes the distortion. The predistortion applied is the inverse of the distortion caused by the amplifier. Thus the output of the amplifier will be linear with respect to the signal prior to the application of the predistortion and the passing of that signal through the amplifier. The transmitter illustrated in this document is of the type where a different transmit path is provided for each channel.
Reference is also made to the following papers: Linearization of Multi-Carrier Power Amplifiers by Johansson, Mattsson and Faulkner (IEEE 1993); Linearization of RF multicarrier amplifiers using Cartesian Feedback by Johansson and Sundstrom (Electronics Letters Jul. 7, 1994); and Linearization of Wideband RF Power Amplifiers by Johansson and Faulkner. These documents disclose a transmitter which uses Cartesian feedback to correct for the non linearity of the amplifier. A signal from the output of the power amplifier is fedback and demodulated. The demodulated signal is subtracted from the input signals to provide an error signal. The error signal is used to drive the modulator and the amplifier. The feedback is performed in the analogue domain. In these documents, a separate Cartesian feedback module is provided for each channel. It should be appreciated that Cartesian feedback is by its nature a narrow band linearization technique.